Hi Daz,
I had this problem recently on my 16V. It was the 'o' ring that seals the top end of the oil cooler take off that had flattened and so was not sealing effectively. You should'nt use a filter wrench to tighten up the oil filter or you risk crushing the oil cooler adaptor .I also changed the metal gasket between the adaptor and the side of the block, just in case. Its now all oil tight again.